The inner surface of a container (1) is treated in a plasma enhanced process, e.g. is cleaned, activated, sterilized or coated, whereby the plasma is confined to a narrow space between an inner member (3) and the inside surface of the container (1). The inner member (3) is adapted in shape to the inside shape of the container, it is hollow and has a porous or perforated wall of a non conductive material or transparent for microwaves, depending on theprocess. The inner member (3) is connected to the supply of the gas and/or vapour mixture which is pressed through its wall into the space between inner member (3) and inside surface of the container (1). The porosity or perforation of the inner member (3) is designed such that when pressing the gas and/or vapour mixture through its wall it causes a pressure drop large enough for the pressure (p2) inside the inner body to be too high for plasma ignition and the pressure (p1) outside of the inner member to be adequately reduced for plasma ignition. For being applicable for containers with a narrow opening, the inner member (3) is preferably made of an elastically extensible material and is designed such that it is inflatable by the pressure difference.